The present invention relates to a digital image comparing system.
Digital image processing systems are known, which provide for comparing two similar images, e.g. two images of similar objects, or two images of the same scene recorded at different times.
More specifically, such systems provide for comparing a detected image with a reference image to determine any differences between the two, and are used for quality control, e.g. automatic surface control (of printed circuits, fabrics, etc.) and for controlling printing processes, e.g. in the case of paper currencies, for determining any difference between a test and reference bank note, and rejecting the test note when the differences detected exceed a given threshold value.
The above systems are also used as part of automatic cartography systems, for grouping different images (e.g. satellite-recorded images of the earth's surface) by superimposing common portions of adjacent images, or for comparing images of the same scene recorded at different times.
The image processing method normally employed by known systems of the above type comprises three main steps: a first step in which reference points are selected in a first image; a second step in which reference points corresponding to those selected in the first image are selected in a second image; and a third step in which the parameters of the transformation function relating corresponding reference points in the two images are calculated, e.g. by means of a least mean square algorithm.
The reference points may be selected manually or semiautomatically. In the first case, the operator selects the reference points in the first image manually, and the selection made by the operator may be improved and refined with the aid of the computer. This method provides for selecting an extremely limited number of points.
In the second case, the image is searched for points defining reference marks or figures with given recognizable characteristics, e.g. intersecting lines, widely curving contour lines, centers of enclosed regions, figure angles, etc. The drawback of this type of selection, however, is the highly complex computing and processing work involved, and the large number of numeric operations required to calculate the function relating the corresponding reference figures or reference marks in the two images.